Lasers aren’t just useful for entertaining cats or pointing out features of powerpoint slides. They can also drill holes on icy extraterrestrial bodies from comets to Mars polar caps. At least according to a new paper in Acta Astronautica by researchers at the Technical University of Dresden, who describe a new laser drill for use on icy surfaces throughout our solar system.
The problem the system is trying to solve is a simple one – drilling holes into ice on other planets and comets is typically done with a “cryobot” – essentially a hot stick designed to melt the ice through thermal contact. There are several problems with this system, with one of the biggest being a power requirement. A cryobot can require kilowatts of energy. A typical lander’s radioisotope thermal generator can only provide a few hundred watts. Landing a kilowatt level power system somewhere else in the solar system is extraordinarily difficult and expensive.
There are other problems too. To get really deep into the ice would require a longer poker – and therefore more material. Storing all that material is difficult, and it also adds to the overall weight of the lander. Even the environment itself works against this method. Since most of the drilling would be done in a complete vacuum, the ice would sublimate into water vapor rather than transfer into liquid, making the thermal contact between the cryobot and the ice its trying to melt tenuous at best.
Fraser makes the case for going to some of the icy moon of Jupiter.
So why not laser the ice instead? Lasers solve many of the problems of a cryobot. They are physically small, and can be very low power. They don’t require a physical interface with the ice, as the laser can be directed onto any surface. And even better, the sublimated water could float out of the borehole and bring trapped particles up to the lander for analysis without being bogged down by a bulky metallic probe.
To prove out the idea, the researchers, led by Martin Koßagk of the Institute for Aerospace Engineering, set up a test experiment in a vacuum chamber with a 1550nm infrared laser. They chose that wavelength because it is particularly strongly absorbed by ice, allowing more of the energy to be transferred directly to the material to be melted.’
They ran experiments on three different types of ice, representing different kinds of ice expected to be found throughout the solar system. The first type was standard “clear” ice, and they were able to achieve a drill rate of approximately 1 meter per hour with a little under 20W of power supplied to the laser. All of these depth measurements were provided with a visible light rangefinder that was mounted in line with the laser.
Some of the sample holes created by the laser. Credit – M. Koßagk et al
“Granular” ice was their next target – these are ice grains rather than a solid block that are more typical of what we would expect to find on a frozen moon like Enceladus. In this environment the laser worked even better, with a drill speed of 1.7 m/h and a power consumption of only 12.7W. This improved speed is likely because of the lower density of the grains compared to the bulk ice.
A final even more impressive test was a series done with “dusty” ice, where “dust” (i.e. non-volatile material like rock) made up 50% or more of the sample. Since the laser only needed to sublimate the ice portion of the sample, and that sublimation would forcibly eject much of the material needed back up the borehole, the system was able to achieve much faster results with this type of sample. With 50% dust, the system was able to go about 3.1 m/h with around 10W of power.
Those are impressive speeds, especially when considering that the system could work all the time, leading to impressively deep holes relatively quickly. That being said, there are some drawbacks, and some further works that needs to be done.
Image of the borehole the laser dug through some of the samples. Credit – M. Koßagk et al
The borehole that was drilled was only about 6.15mm wide – not very much room to get any sort of probe or anything else down under the surface of the ice. There’s also a risk that the pressure at the bottom of a deep hole could build up enough that the ice begins to melt rather than sublimate, decreasing its effectiveness by essentially heating up melted water rather than ice directly.
Boreholes on Earth are known to “squeeze” shut after a certain depth, and while that remains a potential issue on other planets, their lower gravity would likely mean they would stay open for deeper than they would typically on Earth. However, one of the system’s advantages – the fact that dust is expelled from within the borehole – also has a down-side. It can coat the mirrors used to direct the laser, decreasing their efficacy. Any fully scaled up system would require a method to decrease that contamination.
Ultimately this is a great step in a new direction for digging deep into ice bodies throughout our solar system. Though it really was only a small step – they successfully drilled to only about 25 cm. In the long run, some kind of borehole system will be needed for icy body missions in the future, and a laser seems to be a very viable option, with some further testing and tweaking. Someday soon a laser might be melting its way through the surface of one of the solar system’s most interesting inhabitants. Unfortunately there probably won’t be any cats there to chase it.
Learn More:
M. Koßagk et al – First tests of a laser ice drill for the exploration of interplanetary ice and icy soils
UT – Testing a Probe that Could Drill into an Ice World
UT – ARCHIMEDES: Digging into the ice on Europa with lasers
UT – CoRaLS Instrument Could Identify Buried Lunar Ice